In a DWDM (Dense Wavelength Division Multiplexing) system, one of key technologies is an optical amplifier using an EDFA (Er3+ Doped Fiber Amplifier). The EDFA has a gain profile as shown in FIG. 5, for example. In other words, the profile of EDFA is not flat since high gain is shown at the wavelengths ranging from 1530 to 1560 nm.
The profile lines a through e on FIG. 5 are shown in ascending order of the pump light level input to the EDFA. In other words, among these profile lines a through e, the profile line a shows a gain profile obtained when the pump light level is the smallest while the profile line e shows a gain profile obtained when the pump light level is the largest.
Then, in order that the EDFA is applied to the DWDM system, flattening of the EDFA gain profile has been pursued. This gain flattening is carried out by applying an optical gain flattening filter which has a loss profile reverse to the EDFA gain profile.
In addition, there has been proposed lattice filter type gain flattening by the use of a multistage Mach-Zehnder interferometer (MZI) (see Zinguji et al, “design of lattice type gain equalizer based on Lawson Rule” (2001 IEICE General Conference C-3-50).
Further, as another example of gain flattening with use of PLC, there has been proposed optical transversal filter type gain flattening (see for example, A. Ranalli and B. Fondeur, “PLANARTAPPED DELAT LINE BASED, ACTIVELY CONFIGURABLE GAIN-FLATTENING FILTER” Proc. ECOC, Paper 7-1-2, 2000)